Many industrial processes produce waste solutions that are laden with heavy metals. These industrial processes include, among others, electroplating, galvanizing, anodizing, chelating, metal finishing, printed circuit board (PCB) manufacturing, semiconductor, magnetic disk manufacturing, mining operations, photo processing, fungicide manufacturing, food preparation, paper and pulp, textile, and oil refining. Examples of the heavy metal contaminants include copper, iron, gold, lead, nickel, silver, tin, zinc, chromium, cadmium, and arsenic. The presence of these heavy metals causes the waste solutions to be highly toxic and oftentimes corrosive and flammable.
In photographic processing waste solutions, the heavy metal ions are tightly bound by chelating agents such as ethylenediaminetetraacetate (EDTA), nitrilotriacetic acid (NTA), and thiosulfate. Waste solutions discharged from plants manufacturing printed circuit boards or the like typically contain heavy metal-chelating agent complexes (e.g., Cu-EDTA) that are derived from chemical copper plating solutions. Waste solutions generated from cleaning metals are produced in the service and maintenance of transportation vehicles and conveyances; military hardware and construction equipment (particularly on wheel and brake parts); aircraft engines; and in other aerospace and military applications. Such “chemical metal cleaning wastes” typically contain, for example, iron, copper, and other metals and heavy metals complexed with a chelating agent, such as EDTA.
Conventional methods for removing heavy metals from waste solutions involve reacting the heavy metals with hydroxide or sulfide compounds, and precipitating the resulting metal hydroxide or metal sulfide from the waste solutions. These methods work fairly well where the heavy metals are present in waste solutions as free ions. However, chelating agents have a high affinity for heavy metals. Thus, when a waste solution contains a chelating agent capable of forming a water-soluble complex with the heavy metal(s), conventional methods for removing the heavy metal(s) are typically ineffective to break up the strong complex of the chelating agent and heavy metal. Therefore, the heavy metal will remain in such a waste solution as part of a soluble complex with the chelating agent.
It has been thought necessary, therefore, to destroy the chelating agent in waste solutions, and thereby release heavy metals as free ions. Several oxidants have been used to destroy chelating agents. For example, U.S. Pat. No. 3,767,572 involves a method for treating photographic processing waste solutions containing heavy metal-EDTA complex by chlorinating the waste solutions with chlorine gas or hypochlorite solution under alkaline conditions to destroy the EDTA chelating agent. U.S. Pat. No. 4,332,687 involves oxidation and destruction of the chelating agent using peroxide and ozone. U.S. Pat. No. 4,846,978 involves that addition of magnesium hydroxide to waste solutions prior to ozonation to provide decomposition of chelating agent. In U.S. Pat. No. 4,289,594, ozonation is used in combination with ultraviolet light to oxidize and destroy the chelating agent. U.S. Pat. No. 4,512,900 describes a method for treating liquid wastes using hydrogen peroxide, followed by ozonation with simultaneous irradiation with ultraviolet light. In U.S. Patent Publication No. 2008/0038169, the waste solution is acidified and oxidized by nitric acid at an extreme temperature and pressure of 200° C. and 700 psig for about 60 minutes, thereby likewise destroying the chelating agent in the waste solution.
Another approach is based on ion replacement by using an excess amount of calcium hydroxide to provide free heavy metal ions. For example, U.S. Pat. No. 4,629,570 relates to a process for removing iron from an aqueous liquid containing soluble iron-EDTA complex. The process involves adjusting pH of the liquid to at least 12.5, adding a soluble calcium salt to facilitate the growth of insoluble iron hydroxide particles, and adding an alkali metal N,N-dimethyl- or N, N-diethyldithiocarbamate to precipitate the iron hydroxide solid from the liquid effluent.
Ion exchange resins may be used to isolate the heavy metals from the waste solutions containing a soluble heavy metal-chelating agent complex. This approach is, however, rather costly and time consuming.